1. Field of the Invention
The invention relates to pressure-sensitive adhesive and adhesive tape, especially a tape having a thick foam or foamlike core and a relatively thin, dense, pressure-sensitive adhesive surface layer which has been selected to provide improved adhesion to specific surfaces. The novel tapes are primarily useful for permanently attaching articles to automotive vehicles.
2. Description of the Related Art
Automotive body side molding usually is permanently mounted either by mechanical fasteners or by a pressure-sensitive adhesive tape having a thick foam or foamlike core and relatively thin pressure-sensitive adhesive surface layers. Among other uses for which pressure-sensitive adhesive tapes preferably have a foam or foamlike core are sealing skylights, attaching simulated mullion bars of glass windows, and attaching medallions to automotive vehicles.
An acrylic foamlike core is provided by a layer of acrylic pressure-sensitive adhesive in which are dispersed microbubbles as disclosed in U.S. Pat. No. 4,223,067 (Levens). The Levens patent uses ultraviolet radiation to polymerize the adhesive of its foamlike layer, but says that strong bonds to certain automotive paint surfaces can be attained only by pressure-sensitive adhesives which cannot be polymerized by ultraviolet radiation. In such event, it may be desirable to apply to one or both of its microbubble-filled faces a layer of unfilled pressure-sensitive adhesive selected for its adhesion to such surfaces. By having a thick foam or foamlike core, a pressure-sensitive adhesive tape better conforms to uneven surfaces.
U.S. Pat. No. 4,364,972 (Moon), which issued subsequent to the Levens patent, concerns an acrylic pressure-sensitive adhesive that can be polymerized by ultraviolet radiation and yet "adheres strongly to automotive paints and to rubber or plastic foam layers" (col. 2, lines 44-46). The Moon pressure-sensitive adhesive comprises a copolymer of acrylic acid ester of nontertiary alcohol and N-vinyl-2-pyrrolidone in an amount comprising from 15 to 50 parts by weight of the total monomers. The Moon pressure-sensitive adhesive has been used as a dense, unfilled surface layer of a composite tape having a foamlike core. Even though that surface layer adheres strongly to automotive paints that were in general use when the Moon patent was filed, automotive manufacturers have indicated that neither that composite tape nor any other pressure-sensitive adhesive tape on the market adheres as strongly as desired to the high-solids automotive paint systems that are coming into widespread use in order to reduce air pollution resulting from evolution of organic solvents into the atmosphere. For a discussion of high-solids automotive paint systems see "American Paint & Coatings Journal", Mar. 18, 1985, pages 54-58.
3. Other Related Art
Because the present invention employs tackifiers, attention is directed to prior teachings of the use of tackifiers in pressure-sensitive adhesive tapes. The earliest pressure-sensitive adhesive tapes were based on mixtures of rubbers and tackifier resins, and tapes employing these rubber-resin pressure-sensitive adhesives are still widely sold. One family of such tapes is disclosed in U.S. Pat. No. 3,681,190 (Dahlquist). Another class of pressure-sensitive adhesive tapes has been developed from polymers which require no tackifier resin, being inherently tacky and pressure-sensitive adhesive, as exemplified by copolymers of (a) acrylic acid ester of nontertiary alkyl alcohol and (b) copolymerizable monomer having a polar group such as acrylic acid. Such a pressure-sensitive adhesive is disclosed in U.S. Pat. Re. No. 24,906 (Ulrich) and is here sometimes called an "acrylic pressure-sensitive adhesive".
A large number of patents, which issued after the Ulrich patent, concern acrylic pressure-sensitive adhesive tapes, but most of them say nothing about tackifiers. One which does, U.S. Pat. No. 4,243,500 (Glennon), uses ultraviolet radiation to copolymerize a coating of a mixture of an unsaturated acrylate ester monomer, a thermoplastic tackifier resin, and an elastomer such as a non-crystallizing elastomeric block copolymer. Another is U.S. Pat. No. 4,500,683 (Hori et al.). A third patent that discloses an acrylic pressure-sensitive adhesive including tackifiers is U.S. Pat. No. 4,456,741 (Ames) in which the acrylic pressure-sensitive adhesive comprises a terpolymer of n-butyl acrylate, N-vinyl-2-pyrrolidone, and styrene. The Ames patent says that this pressure-sensitive adhesive terpolymer "may be used alone or in admixture with other materials such as polyterpenes" and then names several, all of which are tackifiers (col. 2, lines 47-53), but does not otherwise mention tackifiers. Neither does it mention any effect from employing a tackifier, and no tackifier is used in any of the examples. The Ames pressure-sensitive adhesive can be hot-melt coated without solvent, thus avoiding evolution of volatiles in the coating operation.
Also concerned with tackified acrylic pressure-sensitive adhesives is European patent application No. 85,307,696.6 (Publication No. 0,183,368, June 4, 1986) which specifically concerns pressure-sensitive adhesives that can be hot-melt coated without solvent.
U.S. Pat. No. 4,645,711 (Winslow et al.) discloses tackified acrylic pressure-sensitive adhesive tapes which are removable from automotive paint after paint baking and hence are useful as automotive masking tapes.
Except for the Winslow patent, none of the above citations in this "Other Related Art" section states that any of its pressure-sensitive adhesives would be useful by reason of providing enhanced adhesion to automotive paints or any other paints.
The above-cited Moon patent, which concerns pressure-sensitive adhesives designed especially to provide enhanced adhesion to automotive paints, teaches that tackifiers can be blended into the photoactive mixtures of monomers from which those pressure-sensitive adhesives are photopolymerized, but warns that "the addition of any such material adds complexity and hence expense to an otherwise simple, straightforward, economical process and is not preferred except to achieve specific results" (col. 6, lines 3-12). The Moon patent does not exemplify this teaching. However, the introduction of a tackifier into a photopolymerizable mixture of monomers often interferes with the polymerization and prevents the attainment of the desired adhesive and cohesive properties.